The present invention relates to a distillation column and method for promoting a uniform vapor flow through the distillation column. More particularly, the present invention relates to such a distillation column and method in which the distillation column employs an annular collector and a liquid redistributor to redistribute liquid between beds of structured packing. Even more particularly, the present invention relates to such a distillation column and method in which a truncated bed of structured packing is employed above the annular collector to promote a more uniform distribution of vapor flow.
Mixtures are separated within distillation columns by contacting an ascending vapor phase of the mixture with a descending liquid phase of the mixture. The ascending vapor phase of the mixture becomes ever more enriched in higher volatility mixture components as it ascends and the descending liquid phase becomes ever more enriched with the lower volatility mixture components as it descends. In order for the distillation to occur, the vapor and liquid phases of the mixture must be brought into intimate contact by mass transfer elements which can be in the form of structured packing. Structured packing consists of corrugated sheets of material that are vertically oriented and arranged such that the corrugations crisscross one another. The vapor phase rises through the corrugations and the liquid phase descends as a film along the surface of the packing.
The height of a particular packing that is necessary to effectuate a particular distillation is determined by experimental and/or other types of theoretical data. Such predictive performance data for a particular packing used for separating a particular mixture is referred to as height equivalent to a theoretical plate which is abbreviated as HETP. In order to reliably use such data to design a column, the liquid at any level of the column must have a fairly constant composition across the column and the vapor must have a uniform flow across the column and therefore, the packing to contact the descending liquid. If this does not occur, then the packing operates with less efficiency and therefore a higher HETP.
In order to ensure a constant composition of liquid across the column, liquid redistributors are placed between packing beds. There are many designs for such liquid redistributors. Some of such liquid redistributors employ an annular collector to funnel liquid to the redistributor. An example of such a liquid redistributor is in the form of an open-ended container, perforated at the bottom for liquid to drip onto the underlying packing. The annular collector is in the form of an annulus with a central open area. Another example is a chevron-type collector that is employed to direct liquid to a channel distributor., Such annular collectors can become particularly troublesome because they act as a constriction to produce an accelerated, central flow of vapor that is not evenly distributed across the column. This produces a loss in the predicted performance of the packing overlying the annular collector.
As will be discussed, the present invention provides a distillation column and a method ensuring uniform vapor flow across the distillation column when annular collectors are used throughout the packing within the column to perform as predicted.
The present invention provides a distillation column that has a plurality of beds of structured packing located within a column shell. The structured packing is configured to contact liquid and vapor phases of a mixture to be distilled within the distillation column. At least one liquid redistributor is located between the beds of structured packing to redistribute liquid of the liquid phase from an overlying bed of structured packing to an underlying bed of structured packing. An annular collector is located above the at least one liquid redistributor to direct said liquid to said at least one liquid distributor. The annular collector acts as a constriction to produce a central vapor flow of said vapor phase. This central flow of vapor would tend to lessen the efficiency of the packing because the vapor would not be evenly distributed across the packing. In order to overcome this, the present invention provides a truncated bed of structured packing included in the plurality of beds of structured packing that is located directly above the annular collector to promote a more uniform distribution of the central vapor flow in a transverse direction of the column shell. The truncated bed of structured packing is fabricated from layers of packing each having a height less than that of packing layers of an overlying bed of structured packing.
As may be appreciated, the truncated bed of structured packing will increase column height and therefore cost. Therefore, the height of the layers of packing that form the truncated bed are preferably half as high as those of the overlying bed of structured packing. As may be appreciated, the constriction of the liquid distributor produces a reduction in the cross-sectional flow area of the column. As a result, the vapor velocity increases. This can produce premature flooding in the truncated bed of structured packing. Therefore, the truncated bed of structured packing is selected to have a packing density less than that of the overlying bed of structured packing.
In another aspect, the present invention provides a method of promoting a uniform vapor flow through at distillation column having beds of structured packing, at least one liquid redistributor located between the beds of structured packing, and an annular collector is located above the at least one liquid redistributor to direct said liquid to said at least one liquid distributor. The annular collector acts as a constriction to produce a central vapor flow of said vapor phase. In accordance with the method, the central vapor flow from the annular collector is introduced into a truncated bed of structured packing, included in the plurality of the beds of structured packing and fabricated from layers of packing each having a height less than that of packing layers of an overlying bed of structured packing.
It should be pointed out that the present invention applies to a bed of structured packing of any type. The term, xe2x80x9cstructured packingxe2x80x9d as used herein and in the claims means a packing that consists of multiple sheets of corrugated material in which the corrugations crisscross one another and the sheets are arranged in a vertical orientation within the column. The term, xe2x80x9cbedxe2x80x9d as used herein and in the claims means an arrangement of structured packing in multiple layers in which the sheets of structured packing of one layer are orientated at right angles to the sheets of structured packing of an adjacent layer. Each xe2x80x9clayerxe2x80x9d, as used herein and in the claims is fabricated by plurality of side-by-side sheets of corrugated material.